NTC ceramics can be used, for example, for insurance current limited (ICL). They are relatively low-impedance semiconductors which, when connected in series with a load resistance (for example a device), can attenuate a switch-on current surge which occurs. As a result of Joule heating, the resistance of an NTC ceramic can rapidly be reduced further during operation with a sufficiently high activation energy, so that high current conduction is achieved within a short time.
The temperature dependency of the resistivity ρT of the NTC ceramic can be described to a good approximation in a particular temperature range, for example between 25° C. and 100° C., by the formulaρT=ρ25° C.·eB/t.
Here, ρ25° C. is the resistivity at the reference temperature, the so-called rated temperature, and the B constant according to the relation EA=k B is an expression of the activation energy of the charge transport EA. Here, k stands for Boltzmann's constant.
The resistivity ρ25° C. and the B constant are thus characteristic quantities of an NTC ceramic and, in a particular temperature range, for example between 25° C. and 100° C., define the vertices of the so-called characteristic curve of an NTC ceramic.
NTC ceramics may, for example, be oxide ceramic semiconductors which contain transition metal cations in neighboring oxidation states at crystallographically equivalent lattice sites. A high volume concentration of mobile electrical charge carriers is thereby formed, the transport of which is excited by the lattice vibrations and takes place between lattice sites which are occupied by transition metal cations in neighboring oxidation states. The coupling of electrical charge carriers to lattice vibrations is also referred to as polaron transport.
The isomorphic incorporation of cations with different valency at crystallographically equivalent lattice sites also causes, with a statistical distribution, a binding energy variable from lattice site to lattice site of the polarons to the lattice sites occupied by cations with neighboring oxidation states so that, owing to such variations, a sufficiently high activation energy is set up which ensures a rapid thermally induced current rise.
Such properties have hitherto been implemented on the basis of oxide systems of the form AB2O4 in phase-homogeneous and phase-heterogeneous mixed phases, which form the spinel structure. In this context, phase-homogeneous means that the constituents of the ceramic based on the spinel structure with the general composition AB2O4 form a solid solution. The mixed phase is phase-heterogeneous when at least two different spinels of different structure exist next to one another or individual constituents are not, or not fully, dissolved and are therefore also present as a constituent in addition to the solid solution.
The oxide systems contain for example cobalt oxide, nickel oxide and copper oxide, respectively in conjunction with manganese oxide as the main constituent. Depending on the composition, manganese occurs in variable proportions as trivalent and tetravalent, cobalt as divalent and trivalent, copper as divalent and optionally also monovalent, and nickel as divalent.
The different values of the crystal field stabilization energy, to which the transition metal cations in their various oxidation states are subject at the octahedron and tetrahedral sites of the spinel structure, give rise to temperature-dependent cation distributions and often also symmetry distortions, which lead to phase-heterogeneous mixed phases of cubic and tetragonal, or rhombic, spinels of different composition. The electrical properties in such cases result from the superposition of properties of the constituents of a phase-heterogeneous structure, which can be adjusted reproducibly in the manufacturing process.